Joe Rankin-Costello pounced on a loose ball in the box to pull one back for Blackburn soon after the restart and Ben Brereton Diaz made it 3-3 in the 63rd minute with a curling shot from 20 yards out.

That sparked a frantic finale as Millwall sought a winner which would salvage a top-six spot, but Blackburn had the better chances.

Brereton Diaz tapped in to round off a counter-attack with just four minutes remaining and cap a remarkable turnaround.

Millwall had started the day in sixth but dropped to eighth as Sunderland moved above the Lions, courtesy of their 3-0 win at Preston, and Blackburn finished seventh on goal difference.

Millwall's win at Blackpool on 28 April meant Gary Rowett's side knew that a final-day victory would be enough to secure a play-off berth, while Rovers - who had slipped to ninth after an eight-match winless run - had to take all three points and hope other results went their way to finish sixth.

The hosts made an aggressive start in front of a raucous crowd and took the lead in the eighth minute when a long throw from Zian Flemming found its way through to Watmore, who crashed in his shot off the crossbar.

Tom Bradshaw thought he had made it 2-0 three minutes later after tapping in after a Flemming free-kick which came back off the left-hand upright, but an offside flag denied the Welsh striker his 18th league goal of the season.

Blackburn levelled when Wharton squeezed in a curling left-footed shot from 25 yards out, yet Millwall appeared to be in full control at half-time after capitalising on defensive errors to net twice in the space of three minutes.

First Watmore converted from a narrow angle after a mix-up between Harry Pickering and goalkeeper Aynsley Pears and then Burke picked up an errant pass and advanced forward to shoot low into the bottom left-hand corner.

Rovers return stuns Lions

However, Blackburn were a team transformed after the break and got back into the game when a Brereton Diaz free-kick was palmed away by George Long and, after a follow-up effort from Callum Brittain had been blocked, Rankin-Costello poked in.

Brereton Diaz ended his eight-game goal drought in the league to make it 3-3 and almost put Rovers back in front a minute later as Millwall lost their grip on the contest.

The Chile international found space in the box and curled against the right-hand upright before Tyrhys Dolan sent the rebound back across the empty goal and bobbling wide.

Scott Malone then cleared a goalbound header from Brereton Diaz off the underside of his own crossbar, while an increasingly desperate Millwall offered little in attack.

Substitute Tyler Burey did force a save from Pears but Blackburn quickly broke away and Sammie Szmodics centred the ball for Brereton Diaz to tap into an empty net and leave the home crowd stunned into silence.

Ultimately Blackburn's win proved to be in vain and Millwall's poor form in the run-in, which saw them pick up just two wins in their final nine games, means their exile from the top flight will continue into a 34th year.

Millwall players were left to rue their second-half collapse which effectively cost the Lions a place in the play-offs

Millwall manager Gary Rowett told BBC Radio London:

"It was always going to be a little bit lively with what's at stake.

"I thought we counter-attacked and took a few of our moments really well to go 3-1 up. Coming in at half-time it was really [about] playing with discipline and doing the right things.

"A goal was always going to change the feeling in the stadium and among the players.

"The key for me is we went chasing and losing our shape and running out of position - and at that point you need to have a bit of calmness and solidity.

"While there is part of me trying not to be too disappointed - because that is my overriding feeling - the fourth goal didn't really matter as much as the third one as we knew the scores and knew we had to win the game. We couldn't quite manage to do that."